There is currently enormous pressure to develop alternative power sources for vehicles (and other systems needing a portable energy source) to reduce US national dependence on petroleum products as an energy source. The US government and industry has identified hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV) as one method to reduce the US national dependence on petroleum products. For example, a key issue for HEV and, in particular, PHEV is the development of batteries which operate at lower temperatures since a large fraction of vehicles and power sources in the US must be able to function at temperatures in the range of 0° C. and less. Tests have shown that the battery chemistry presently used cannot meet requirements below 0° C., a relatively common winter temperature for the northern half of the U.S. The cause of this problem is not known at this time. For example, recently for HEV's and low mileage PHEV's, the DOE, USCAR, and several manufacturers have identified lithium titanate as a possible anode for these systems for safety and power reasons. However, as with other systems, it has the same low temperature issues.
In typical lithium ion battery systems, the electrolyte is a mixture of organic solvents containing a lithium salt, e.g. 1M LiPF6. For many battery systems the limiting factor in overall performance is the ability of the electrolyte to conduct lithium ions and have stability over the appropriate electrochemical window. Evidence suggests that a limiting factor in improving battery power and improving low temperature performance is the carbonate-based electrolytes presently used, which are optimized for room temperature operation, while low temperatures typically require special additives. In the next generation of hybrid electric vehicle (HEV), battery cost and power are key issues, in addition to the ability to function at a variety of temperatures, including low temperatures.